lnd: integrate Sphinx onion routing when sending/receiving HTLC's

This commit alters the send/receive HTLC pipe line a bit in order to
fully integrate onion routing into the daemon.

The server now stores the global Sphinx router which all active
htlcManagers will used when processing upstream HTLC add messages.
Currently the onion routing private key is static, and identical to the
node’s current identity public key. In the future this key will be
rotated daily the node based on the current block hash.

When sending a payment via the SendPayment RPC, the routing manager is
now queried for the existence of a route before the payment request is
sent to the HTLC switch. If a path is found, then a Sphinx onion packet
encoding the route is created, then populated within the HTLC add
message.

Finally, when processing an upstream HTLC add request, the sphinx
packet is decoded, then processed by the target peer. If the peer is
indicated as the exit node, then the HTLC is queue’d to be settled
within the next state update.

server.go

@@ -9,6 +9,7 @@ import (
 	"sync/atomic"
 
 	"github.com/btcsuite/fastsha256"
+	"github.com/lightningnetwork/lightning-onion"
 	"github.com/lightningnetwork/lnd/chainntnfs"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/lndc"
@@ -58,6 +59,8 @@ type server struct {
 
 	utxoNursery *utxoNursery
 
+	sphinx *sphinx.Router
+
 	newPeers  chan *peer
 	donePeers chan *peer
 	queries   chan interface{}
@@ -95,13 +98,16 @@ func newServer(listenAddrs []string, notifier chainntnfs.ChainNotifier,
 		invoices:      newInvoiceRegistry(chanDB),
 		lnwallet:      wallet,
 		identityPriv:  privKey,
-		lightningID:   fastsha256.Sum256(serializedPubKey),
-		listeners:     listeners,
-		peers:         make(map[int32]*peer),
-		newPeers:      make(chan *peer, 100),
-		donePeers:     make(chan *peer, 100),
-		queries:       make(chan interface{}),
-		quit:          make(chan struct{}),
+		// TODO(roasbeef): derive proper onion key based on rotation
+		// schedule
+		sphinx:      sphinx.NewRouter(privKey, activeNetParams.Params),
+		lightningID: fastsha256.Sum256(serializedPubKey),
+		listeners:   listeners,
+		peers:       make(map[int32]*peer),
+		newPeers:    make(chan *peer, 100),
+		donePeers:   make(chan *peer, 100),
+		queries:     make(chan interface{}),
+		quit:        make(chan struct{}),
 	}
 
 	// If the debug HTLC flag is on, then we invoice a "master debug"